1. Field of the Invention
The present invention relates generally to firing mechanisms for actuation wellbore tools, and specifically to mechanisms for actuating electrically-actuable wellbore tools.
2. Description of the Prior Art
A variety of conventional wellbore tools require the selective application of an electrical triggering signal to initiate changes in modes of operation between selected modes of a plurality of modes of operation. Since wellbore tools are generally located at substantial distances within the wellbore, it is not feasible to provide a direct electrical connection from the earth's surface to the wellbore tool. Consequently, most wellbore tools require an actuation mechanism which is sensitive to stimulus which can be controlled from the surface, such as axial force, set down weight, or fluid pressure.
Of course, changes in axial force, set down weight, and fluid pressure can occur accidentally as a wellbore tool is being run into or out of a wellbore. Consequently, a wellbore tool may become inadvertently actuated in an undesirable location, most probably causing serious economic losses.
A variety of conventional wellbore tools which seal, pack, hang, and connect with or between concentrically nested wellbore tubular members are set into position by application of axial forces to the tool, such as, for example, by either lifting up on a tubular string to lessen the load on a tool, or by applying a selected amount of set down weight to the tubular string, to cause selected components to move relative to one another. For example, liner hangers frequently include slip and cone assemblies which are loaded to cause a portion of the assembly to come into gripping engagement with a wellbore selected surface. For alternative example, packers frequently include elastomeric sleeves which are compressed and energized to urge the sleeve into sealing engagement with a selected wellbore surface.
Of course, these types of wellbore tools require that operations usually performed at the surface cause an intended effect at a remote location deep within the wellbore, and in particular require that axial force be transferred effectively over great distances, even in difficult wellbores, such as deviated or spiral-shaped wellbores. Those knowledgeable about wellbore completion operations will appreciate that a force-transmitting tubular string may contact other wellbore tubulars or wellbore surfaces at a number of locations, dissipating the axial setting force which his intended for application at another location, and frustrating completion operations.
Another related problem with the prior art devices is that the wellbore tool may be unintentionally subjected to axial, or other, loads during running of the tool into the wellbore, which may cause unintentional setting of the tool in an undesirable or unintended location. Since many wellbore tools, such as liner hangers or packers, are designed to permanently lock in a set position, such an accidental setting can result in extremely expensive and time-consuming retrieval operations.